Classes and Inheritance - Illustrated C# 2005 - page 157

Inheritance Between Assemblies
So far, I have been declaring derived classes in the same assembly where the base class is
declared. But C# also allows you to derive a class from a base class defined in a different assem-
bly. To do this, the following must be true:

The base class must be declared public , so that it can be accessed from outside its assembly. ￿

￿ You must include a reference in your Visual Studio project, to the assembly containing

the base class.

To make it easier to refer to the classes and types in the other assembly, without using their
fully qualified names, place a using directive at the top of the source file, with the namespace
containing the classes or types you want to access.

■ Note Adding a reference to the other assembly and adding a using directive are two separate things.
Adding the reference to the other assembly tells the compiler where the required types are defined. Adding
the using directive allows you to reference other classes without having to use their fully qualified names.
Chapter 10 covers this in detail.

For example, the following two code segments, from different assemblies, show how easy
it is to inherit a class from another assembly. The first code listing creates an assembly that
contains the declaration of a class called MyBaseClass , which has the following characteristics:

It is declared in a source file called BaseClass.cs , and inside a namespace declared as
BaseClassNS .

￿

￿ t is declared public , so that it can be accessed from other assemblies.

It contains a single member, a method called PrintMe , that just writes out a simple mes-
sage identifying the class.

￿

// Source file name BaseClass.cs
using System;
Namespace containing declaration of base class
↓
namespace BaseClassNS
{
Declare the class public, so it can be seen outside the assembly.
↓
public class MyBaseClass {
public void PrintMe() {
Console.WriteLine("I am MyBaseClass");
}
}
}